Method of producing polymethacrylimide foams

ABSTRACT

The invention relates to an improved method for producing foamed material, especially poly(meth)acrylimide foams, which are foamed from polymer plates produced according to the casting method. The two-step method consists of a pre-heating step and at least one foaming step. The product obtained has a significantly smaller compressive strain, measured according to DIN 53425(ASMD 621), than prior art products.

FIELD OF THE INVENTION

[0001] The invention relates to an improved method for the preparationof foams, in particular poly(meth)-acrylimide foams, which are foamedfrom polymer sheets produced by the casting method. The two-stage methodconsists of a preheating step and one or more foaming steps.

DISCUSSION OF BACKGROUND

[0002] Polymethacrylimide foams have long been known and, owing to theirexcellent mechanical properties and their low weight, are widely used,in particular in the production of multilayer materials, laminates,composites or foamed composites. Prepregs comprising polymethacrylimidecore materials are frequently bonded here.

[0003] For example, they are used in aircraft construction, inshipbuilding as well as in automotive construction. For many of thesenumerous applications, they have to meet technical requirements laiddown in statutory provisions and a number of other regulations.

[0004] The present invention relates to the area of the polymer blocksproduced by the casting method and polymethacrylimide foams preparedtherefrom. Here, the monomers methacrylic acid and methacrylonitrile areintroduced between two plane-parallel plates —generally glass plates.After the polymerization, the polymer sheets obtained are foamed in afurther, separate method step.

[0005] The method relevant in production technology is based on foamingin a hot-air oven, which is to be referred to below as the hot-airmethod. The polymer sheets are introduced suspended in aforced-circulation oven, transported through said oven by aself-sustaining traction system and discharged at the end as foamsheets. The distance covered by the sheets in the oven is referred tobelow as L. The foaming time is thus defined by the length L of the ovenand the constant travelling velocity V of the transport system in thetotal oven. The oven throughput depends not only on its length L and thetravelling velocity V of the transport system but also on the timeinterval t and hence also the geometric spacing a of the sheets withwhich the latter are introduced into the oven. Since the sheets aregreatly distorted during the foaming method, the spacing a must belarger than b/n so that the sheets cannot touch one another during thefoaming and thus become damaged. b is defined as the length of the sidefrom which the sheet is suspended and which the sheet has when it hasbeen foamed. The content of this publication is limited to the methodstep comprising foaming.

[0006] DE 3 630 960 describes a further method for the foaming of theabovementioned copolymer sheets from methacrylic acid andmethacrylonitrile. Here, the sheets are foamed with the aid of amicrowave field, and this is therefore referred to below as themicrowave method. It must be ensured here that the sheet to be foamed orat least its surface must be heated beforehand up to or above thesoftening point of the material. Since of course the foaming of thematerial softened by the external heating also begins under theseconditions, the foaming method alone cannot be controlled by theinfluence of a microwave field but also must be controlled from theoutside by accompanying heating. Thus, a microwave field is coupled withthe usual one-stage hot-air method in order to accelerate the foaming.However, the microwave method has proved too complicated and thereforenot relevant in practice and is not used today.

[0007] WO 90/2621 describes a foam obtained from methacrylic acid andmethacrylonitrile, acrylamide as a comonomer preventing prematureformation of precipitates during the polymerization. The foam formed isvery uniform and the product has no internal stresses.

[0008] DE 197 17 483 describes a method for the preparation ofpolymethacrylimide foams to which 1-5% by weight, based on the monomermixture, of MgO are added. Foams having substantially improvedthermomechanical properties are obtained.

[0009] DE 196 06 530 describes the addition of a flameproofing agent bymeans of polymethacrylimide foams.

OBJECT

[0010] In order to make ROHACELL more attractive for existingapplications, it is necessary to optimize its material properties. Heatof reaction evolved during the foaming leads to a temperature gradientin the foamed sheet and therefore also to a location-dependent densityin the sheet. As a result of this, the mechanical characteristics of afoam sheet likewise depend on the sampling location, since the densityis known to have a considerable effect on mechanical properties, suchas, for example, compressive strength or creep behaviour. The heat ofreaction evolved can lead to cracking and hence to the destruction ofthe material in the production of low densities. It has now been foundthat the abovementioned disadvantages can be avoided by the methodfound. For this purpose, a more efficient preparation is to be ensuredby an associated increase in the throughput.

[0011] Achievement

[0012] Surprisingly, the object described above can be achieved bydividing the hot-air method into two separate hot-air processes. Insteadof two hot-air processes, it is also possible to combine three or moreprocesses. In the first hot-air process, the sheet to be foamed ispreheated in a hot-air oven below the actual foaming temperature of thematerial. The linear regression of the temperature increase as afunction of time gives a mean linear heating rate of 0.001-10 K/min,preferably 0.01-5 K/min and particularly preferably 0.1-1 K/min.

[0013] The linear regression of the temperature increase is alsoreferred to as the temperature ramp. The hot sheet is transported fromthe preheating oven into the actual foaming hot-air oven. The foaminghot-air oven has the temperature required for foaming, which is abovethe preheating temperature. The foaming hot-air oven can also consist ofa second oven part of the preheating oven. The temperature profile towhich the sheet is subjected during the foaming is represented by thegrey line in FIG. 1. The high viscosity in the low temperature range ofthe preheating inevitably results in a supersaturated solution of theblowing gas in the polymer. The evolved heat of reaction, which isusually troublesome during the foaming, is uniformly distributed in thepolymer sheet on preheating. Only when the material is heated to thefoaming temperature does phase separation of polymer matrix and blowingagent occur and lead to expansion of the polymer sheet.

[0014] The preheating can be carried out here in the form of atemperature ramp or of a constant preheating temperature. FIG. 1 showsthe difference between the method to date (black line, one-stage hot-airmethod) and the novel method (grey line, two-stage hot-air method) byway of example for the case of a constant preheating temperature.

[0015] Advantages of the method according to the invention:

[0016] In the case of certain formulations, PMI foams have poor creepbehaviour if they are foamed in a one-stage method step. This makesprocessing of such foams as core material possible only to a limitedextent. With the aid of the two-stage hot-air method, the compressionaccording to DIN 53425(ASTMD621) can be reduced to {fraction (1/10)}.

[0017] Furthermore, cracking can occur in foam slabs in the case ofcertain formulations when the one-stage hot-air method is used for theproduction of low densities, which leads to waste. Foam slabs which havecracks owing to imperfect foaming and therefore cannot be used forapplications are to be regarded as waste here. Cracks must not occur.Thus, for example, 40% waste means that 40 out of 100 foam slabsproduced have to be removed and disposed of owing to imperfect foamingand/or cracking. With the aid of the two-stage hot-air method, the wastecan be more than halved.

[0018] Because the actual foaming time can be reduced by upstreampreheating, the travelling velocity V of the transport system in theoven can be increased in the case of a two-stage hot-air method, whichcauses the throughput to increase. FIG. 1 shows, by way of example, thisreduction in the foaming time by the preheating of the polymers, withoutrestricting this effect to the parameters shown there: the foaming timeis reduced to ⅔ of the original foaming time in this example.

[0019] If the uniformly preheated polymer sheet is further heated to thefoaming temperature, no temperature gradient is caused in the sheet byan exothermic reaction and furthermore the temperature gradient due tothe temperature jump to the foaming temperature is itself smaller. Thelarger this temperature jump which the polymer sheet experiences onentering the foaming process, the greater is the temperature gradientcaused thereby and produced in the sheet.

[0020] It is obvious that stress differences and blowing agent pressuredifferences occur in the material, firstly owing to the thermalexpansion and secondly owing to the staggered start of foaming, which islocation-dependent because of the temperature gradient. In the exampleshown in FIG. 1, the temperature jump experienced by the polymer sheeton entering the foaming process is 175 K for the case of the one-stagehot-air method (black line) and only 40 K for the case of the two-stagehot-air method (grey line).

[0021] By means of a suitable (temperature ramp), it is also possibleentirely to avoid a temperature jump. This finally has a majorconsequence for the homogeneity of the foam sheet: the initiallydescribed distortion of the sheets can be suppressed so that thecondition a>b/π no longer need be maintained. This shortens the cycletime t introduced at the outset and, owing to the increase in thethroughput, also has an ecological benefit in addition to the increasednet product with the same oven design.

EXAMPLES

[0022] Comparative example 1

[0023] 330 g of isopropanol and 100 g of formamide were added as blowingagent to a mixture of 5 700 g of methacrylic acid, 4 380 g ofmethacrylonitrile and 31 g of allyl methacrylate. Furthermore, 4 g oftert-butyl perpivalate, 3.2 g of tert-butyl per-2-ethylhexanoate, 10 gof tert-butyl perbenzoate, 10.3 g of cumyl perneodecanoate, 22 g ofmagnesium oxide, 15 g of blowing agent (PAT 1037) and 0.07 g ofhydroquinone were added to the mixture.

[0024] This mixture was polymerized for 68 h at 40° C. and in a chamberformed from two glass plates measuring 50×50 cm and having an 18.5 mmthick edge seal. The polymer was then subjected to a heating programmeranging from 32° C. to 115° C. for 32 h for the final polymerization.

[0025] The subsequent foaming in the hot-air method was carried out for2 h 25 min at 205° C., considerable distortion of the sheet beingobservable during the foaming. In the incompletely foamed state, thesheet curved at one point to such an extent that the two opposite sideswhich are perpendicular to the suspension side touched at one point. Thefoam thus obtained had a density of 235 kg/m³. The compression accordingto DIN 53425 (ASTM D621) was more than 18% at 180° C. and a load of 0.35MPa after 2 h.

Example 1

[0026] The procedure was as described in comparative example 1. However,the hot-air method used was in two stages: preheating was effected for 2h at 140° C. and then foaming for 2 h 75 min at 205° C. Only negligibledistortion of the foamed sheet was observed. The foam thus obtained hada density of 238 kg/m³. The compression according to DIN 53425(ASTMD621) was 12.7% at 180° C. and a load of 0.35 MPa after 2 h.

Example 2

[0027] The procedure was as described in comparative example 1. However,the hot-air method used was in two stages: preheating was effected for 2h at 150° C. and then foaming for 2 h 25 min at 210° C. Only negligibledistortion was observed, which was less than in Example 1.

[0028] The foam thus obtained had a density of 203 kg/m³. Thecompression according to DIN 53425(ASTM D621) was 4.6% at 180° C. and aload of 0.35 MPa after 2 h.

Example 3

[0029] The procedure was as described in comparative example 1. However,the hot-air method used was in two stages: preheating was effected for 2h at 160° C. and then foaming for 2 h 25 min at 215° C. Only negligibledistortion was observed, which was less than in example 2. The foam thusobtained had a density of 208 kg/m³. The compression according to DIN53425(ASTM D621) was 2.9% at 180° C. and a load of 0.35 MPa after 2 h.

Example 4

[0030] The procedure was as described in comparative example 1. However,the hot-air method used was in two stages: preheating was effected for 2h at 160° C. and then foaming for 2 h 25 min at 220° C. Only negligibledistortion was observed, which was similar to that in example 3. Thefoam thus obtained had a density of 168 kg/m³. The compression accordingto DIN 53425(ASTM D621) was 1.3% at 180° C. and a load of 0.35 MPa after2 h.

Example 5

[0031] The procedure was as described in comparative example 1. However,the hot-air method used was in two stages: preheating was effected for 2h at 170° C. and then foaming for 2 h 25 min at 215° C. No distortionwas observed. The foam thus obtained had a density of 199 kg/m³. Thecompression according to DIN 53425(ASTM D621) was 3.5% at 180° C. and aload of 0.35 MPa after 2 h.

Example 6

[0032] The procedure was as described in comparative example 1. However,the hot-air method used was in two stages: preheating was effected for 1h 25 min at 180° C. and then foaming for 2 h 25 min at 210° C. Nodistortion was observed. The foam thus obtained had a density of 218kg/m³. The compression according to DIN 53425(ASTM D621) was 1.6% at180° C. and a load of 0.35 MPa after 2 h.

[0033] Comparative example 1 and examples 1 to 6 clearly show that thecreep behaviour is improved by the preheating. In spite of lowerdensities, a lower compression is observed under identical measuringconditions. On the other hand, it is known to a person skilled in theart that a reduction in the density of a rigid foam results in adeterioration in its mechanical properties, i.e. its creep modulusbecomes smaller and hence the compression greater under identicalmeasuring conditions.

[0034] Comparative example 2

[0035] 42 kg of isopropanol and 47 kg of formamide were added as blowingagent to a mixture of 610 kg of methacrylic acid and 390 kg ofmethacrylonitrile. Furthermore, 0.4 kg of tert-butyl perpivalate, 0.4 kgof tert-butyl per-2-ethylhexanoate, 0.7 kg of tert-butyl perbenzoate,1.03 kg of cumyl perneodecanoate, 2.2 kg of zinc oxide, 1.5 kg ofblowing agent (PAT 1037) and 0.075 kg of hydroquinone were added to themixture.

[0036] This mixture was polymerized for 116 h at 33° C. in chamberswhich were formed from two glass plates measuring 100×200 cm and havinga 30 mm thick edge seal. The polymer was then subjected to a heatingprogramme ranging from 35° C. to 130° C. for 40 h for the finalpolymerization.

[0037] The subsequent foaming in the hot-air method was effected for 2 h30 min at 200° C., considerable distortion of the sheets beingobservable during the foaming. The foam thus obtained had a density of31 kg/m³. However, 40% of the foam thus prepared had to be discarded aswaste, owing to cracking.

Example 7

[0038] The procedure was as described in comparative example 2. However,the hot-air method used was in two stages: preheating was effected for1.5 h at 160° C. and then foaming for 2 min 30 min at 205° C. Nodistortion of the sheets was observed during the foaming. The foam thusobtained had a density of 32 kg/m³. Cracking and the associated materialloss due to waste could be reduced to 5%.

1. A method for production of polymethylacrylimide foamed materials inthe form of blocks or plates by copolymerization of methacrylic acid andmethacrylonitrile as well as of further copolymerizable monomers andadditives if necessary in the presence of radical-forming initiators,postpolymerization and cyclization of the copolymer to the polyimide,and transformation to a foamed material, which comprises: foaming isperformed in a two-stage process step, wherein the first process stepincludes preheating of the polymer to be foamed, while foaming of thematerial takes place in the second process step, and wherein thepreheating is performed in a hot-air oven and the subsequent foaming isperformed in a second hot-air oven or hot-air ovens, and in which theheating rate used to raise the temperature is between 0.001 K/min and 10K/min.
 2. A method according to claim 1, in which the two hot-air ovensor hot-air oven sections used for the two-stage process have differenttemperatures.
 3. A method according to claim 1, in which the temperatureof the hot-air oven used for preheating is lower, while being constantin time, than that of the hot-air oven or hot-air oven section used forfoaming.
 4. A method according to claim 1, in which the temperature ofthe hot-air oven used for preheating is lower, while rising over thecourse of time, than that of the hot-air oven or hot-air oven sectionused for foaming, and wherein the temperature in the hot-air oven usedfor preheating can once again be equal, at the end of the heating cycle,to the temperature in the hot-air oven or hot-air oven section used forfoaming.
 5. A method according to claim 1, in which the heating rateused to raise the temperature is between 0.01 K/min and 5 K/min.
 6. Amethod according to claim 1, in which the heating rate used to raise thetemperature is between 0.1 K/min and 1 K/min.
 7. A method according toclaim 6, in which different heating rates in combination with oneanother can be used for the average linear temperature rise.
 8. A methodaccording to claim 6, in which the final temperature of the temperaturerise can be higher than the temperature that is needed for foaming andthat exists in the hot-air oven used for foaming.
 9. Foamed blocks orplates of polymethacrylimide, obtainable by a method according asclaimed in claim
 1. 10. The use of the foamed blocks or plates accordingto claim 9 as components in sandwich structures.